Did I mention? I ride a two wheel low recumbent. While I went the recumbent way for comfort, a heavy engine and small frontal area are a great recipe for speedy descents as well (yeah I never thought of that !!). Check your tyres and pressures first, look in the mirror for signs of sanity, then remember to keep your mouth closed - flies can't do 90 kph and aren't that tasty!

Let me ignore the climbing side for a bit, and focus on the fun bit. The hill in mind is 12 km long. The upper 6 has some ups and downs and is less useful for tyre testing at speed, though there is an exhilarating 80 to be had for a brief period. The lower 6 km are sufficiently straight and clear for a brakeless run with gradients between 4% and 6% to enable buttock clenching speeds for longish periods. A top section delivers around 80kph over 2 or more kms  finishing around 90 kph, and a lower section delivers 70 kph over a kilometer or so peaking in the high 70s. My best records are for the maximum speed reached in longest and fastest section of the hill. About 10 runs in total.

This year I had a new fatter front tyre to try. The Schwalbe Kojak 35 x 406 folder I felt offered no increase in rolling resistance in my normal speed range of 20 to 30 kph. In fact, I think it's an improvement over the 28 x 406 Stelvio folder of last year. It also offers better grip and durability - a front tyre to adopt for the modest weight increase of 45g. My rear tyre is a Schwalbe Stelvio 28 x 559 folder. It has been a reliable and good rolling tyre for the weight it has to bear.

Last year I clocked a best of 92 kph on the top section, clipping 90 on all descents. This year on the same Schwalbe Stelvio tyres at the same pressures (100psi) I noticed I was hitting 88. This difference of around 4% easily accounted for by this year's roughly 4% lighter engine.

I changed to the new wider Schwalbe Kojak, my new front tyre of choice 35x406 (100psi) and expected to match or better the 88 of the narrower Stelvio. Low and behold, top speed down to 84. Some passes at 82 (loose jersey?).

While not a very scientific comparision, it felt significant on the bike. Similar temperature, similar time of day and with same close fitting cycle clothing. Yeah yeah, there was no measure of air density, wind speed or entry speed for the runs. However, I figured the long roll in at 80ish neutralized the initial entry speed higher up the hill - a casual test and no more. The bike setup was identical except for the changed front tyre.

I reset my computer to the calibrated diameter for each of the tyres. I used the technique of rolling out 3 times the circumference with rider on bike. I got 1444mm for the Stelvio and 1472 for the Kojak.

I believe this ~4% loss is due to the aero effects of the fatter tyre, not the rolling resistance. As I said earlier, I believe the fatter Kojak is the easier rolling tyre at my cruising speeds, so the aero effect at higher speeds might be even greater than the measured 4% - none of this proven. It tends to support the truism about skinny tyres being faster. In the racing world at racing speeds, tyre width does have an effect on wind resistance where it counts!!

I'm staying with the wider Kojak on the front of my recumbent. At my cruising speeds it offers better rolling and improved confidence. It also carries 50% of the load, and a fierce disk brake. The loss of a few percent of aero efficiency at speeds above 50kph is a very insignificant loss in the life of my riding.